Download Series DC Motors

DC Machines
Introduction
One of energy can be obtained from the other form
with the help of converters. Converters that are
used to continuously translate electrical input to
mechanical output or vice versa are called electric
machines.
The process of translation is known as
electromechanical energy conversion.
Electrical
system
e, i
Electric
Machine
Mechanical
system
T, n
Motor
Energy flow
Generator
•An electrical machine is link between an electrical
system and a mechanical system.
•Conversion from mechanical to electrical: generator
•Conversion from electrical to mechanical: motor
Electrical
Machines
DC
machine
AC
machine
Synchronous
machine
Induction
machine
•Machines are called AC machines (generators or
motors) if the electrical system is AC.
•DC machines (generators or motors) if the electrical
system is DC.
Electrical
system
e, i
Coupling
magnetic
fields
Mechanical
system
T, n
Two electromagnetic phenomena in the electric machines:
•When a conductor moves in a magnetic field, voltage is
induced in the conductor.
•When a current-carrying conductor is placed in a
magnetic field, the conductor experiences a mechanical
force.
Electric Machines
Basic Structure
a'
b
c
•The structure of an electric
machine has two major
components, stator and rotor,
separated by the air gap.
Rotor
c'
b'
a
Stator
Stator
• Stator:
Does not move and normally is
the outer frame of the machine.
R
B’
N
Y’
Rotor
Y
S
B
R’
• Rotor:
Is free to move and normally is
the inner part of the machine.
•Both rotor and stator are made
of ferromagnetic materials.
DC Machine
Direct Current (DC)
Machines Fundamentals
• Generator action: An emf (voltage) is
induced in a conductor if it moves through a
magnetic field.
• Motor action: A force is induced in a
conductor that has a current going through it
and placed in a magnetic field
• Any DC machine can act either as a generator
or as a motor.




DC Machine is most often used for a motor.
The major advantages of dc machines are the
easy speed and torque regulation.
However, their application is limited to mills,
mines and trains. As examples, trolleys and
underground subway cars may use dc motors.
In the past, automobiles were equipped with dc
dynamos to charge their batteries.




Even today the starter is a series dc motor
However, the recent development of power
electronics has reduced the use of dc motors
and generators.
The electronically controlled ac drives are
gradually replacing the dc motor drives in
factories.
Nevertheless, a large number of dc motors are
still used by industry and several thousand are
sold annually.
DC Generator Fundamentals
e =(B × v). l
e =B l v sinα cosβ
e = induced voltage, v = velocity of the conductor,
B = flux density and l is the length of the conductor
 - angle between the direction in
which the conductor is moving and the
flux is acting.
β - smallest possible angle the
conductor makes with the direction of,
the vector product, ( v × B) and for
maximum induction,
β = 0. Hence, e = Blv for most cases.
( v × B) indicates the direction of the
current flow in the conductor, or the
polarity of the emf.
DC machine Construction
• Stator: Stationary part of the
machine. The stator carries a field
winding that is used to produce the
required magnetic field by DC
excitation. Often know as the field.
• Rotor: The rotor is the rotating
part of the machine. The rotor
carries a distributed winding, and is
the winding where the emf is
induced. Also known as the
armature.
DC Motors Equivalent circuit
The equivalent circuit of DC Motors (and Generators)
has two components:
• Armature circuit: it can be represented by a voltage
source and a resistance connected in series (the
armature resistance). The armature winding has a
resistance, Ra.
• The field circuit: It is represented by a winding that
generates the magnetic field and a resistance connected
in series. The field winding has resistance Rf.
Classification of DC Motors
• Separately Excited and Shunt Motors
Field and armature windings are either connected
separate or in parallel.
• Series Motors
Field and armature windings are connected in series.
• Compound Motors
Has both shunt and series field so it combines
features of series and shunt motors.
Separately Excited DC Motors
– The armature winding supplies the load.
– The field winding is supplied by a separate DC source whose
voltage is variable.
– Good speed control.
Ea
Shunt DC Motors
– The armature and field windings are connected in parallel.
– Constant speed operation.
Ra
Ia
Rf
Ea
Pout
m
It
If
Vt
Series DC Motors
– The armature and field winding are connected in series.
– High starting torque.
Ra
Rf
Ea
Pout
m
It
Vt
Compound dc motors
Ra
Ia
Rfs
Ea
Pout
m
It
Vt
Rfp
Ifp
Power Flow and Losses in
DC Motors
Protational
VtIt
VaIt
VaIa
EaIa
Pout
Pinput
It 2Rsr
It 2Rt
Ia 2Ra
Speed Control of DC Motors
Speed can be controlled by varying:
1) Armature circuit resistance using an external
resistance Ra Ext.
2) IF can be varied by using an external resistance Radj
in series with Rf to control the flux, hence the speed.
3) The applied voltage to the armature circuit resistance,
if the motor is separately excited
Comparison of DC Motors
Shunt Motors: “Constant speed” motor (speed regulation is very
good). Adjustable speed, medium starting torque.
Applications: centrifugal pump, machine tools, blowers fans,
reciprocating pumps, etc.
Series Motors: Variable speed motor which changes speed drastically
from one load condition to another. It has a high starting torque.
Applications: hoists, electric trains, conveyors, elevators, electric cars.
Compound motors: Variable speed motors. It has a high starting
torque and the no-load speed is controllable unlike in series motors.
Applications: Rolling mills, sudden temporary loads, heavy machine
tools, punches, etc